1. Field of the Invention
The present invention relates to mixing boxes used in heating, ventilation and air conditioning (HVAC) systems to mix recycled air from within a building with fresh air from outside the building.
2. Prior Art Statement
Most all modern office buildings, shopping centers and other large buildings that are typically occupied by people have HVAC systems that regulate the temperature and the quality of the air that is circulated within that building. In addition to either heating or cooling the air within a building, HAVC systems must also introduce fresh air into the air being circulated. The amount of fresh air that must be introduced into a particular building is subject to numerous engineering standards and governmental regulations.
Seldom is the fresh air outside a building the same temperature as the air being circulated within that building. Consequently, when fresh air is introduced into the ventilation system of a building, that fresh air must either be heated or cooled to the temperature of the air being circulated within the building.
In order to mix recycled air with fresh air, both the recycled air and the fresh air are passed through a mixing box. In the mixing box, the two air flows are given the opportunity to mix. The mixed air is then either heated or cooled to the requirements of the building. A key to an efficient HVAC system is the ability to introduce fresh air into the ventilation cycle while minimizing the energy required to either heat or cool the newly introduced air. Although a number of air mixers have been developed for bringing together and mixing fresh air with recycled air, such air mixers tend not to be very efficient in thoroughly mixing all the air. Furthermore, due to the air flow requirements of many large buildings, air mixers are commonly made quite large. The air mixers, therefore, take up a substantial amount of space.
An air mixing chamber is typically the first part of a HVAC unit. The mixed air from the mixing chamber is then passed through heat exchanger coils that are used to cool the mixed air during warm weather. A problem with such a configuration commonly occurs in the winter, during cold temperature conditions. In the winter, the air conditioning unit of the HVAC system is dormant. Consequently, the water or other fluid held within the heat exchanger coils does not circulate as the mixed air flows past the heat exchanger coils. During period of cold weather, the temperature of the fresh air outside a building may be significantly colder than the air circulated within the building. For example, the air on a winter day may be 10 degrees, while the temperature within a building may be 70 degrees. A temperature gradient of 60 degrees therefore exists between the external fresh air and the recycled internal air.
When a large temperature gradient exists between the fresh air and the recycled air, the two temperatures of air tend to stratify within the confines of an air mixer. That is, when the cold fresh air and the warm recycled air are introduced into an air mixer, the cold air tends to sink to the bottom of the mixer and the warm air rises to the top of the mixer. Due to the large temperature gradient between the two flows of air, very little mixing of the air flows actually occurs.
Due to the stratification of the warm and cold air, the temperature of the air at the bottom of the air mixer is little warmer than the external air drawn into the air mixer. The air exiting the air mixer is therefore not well mixed and is not at the proper temperature.
In the worst case scenario, as the cold layer of stratified air in the air mixer flows past the heat exchanger coils next to the air mixer, the cold air can freeze the fluid in the heat exchanger coils and damage the heat exchanger coils. To prevent this scenario from happening, temperature sensors are placed near the heat exchanger coils. If the temperature sensor detects an air temperature that can damage the heat exchanger coil, the unit is automatically shut down. Consequently, on unusually cold days in the winter, it is not uncommon for many different HVAC units to shut down due to poor air mixing.
Various solutions have been proposed in the past to prevent air stratification in an air mixer and to prevent the damage that it can cause to the system. For example, glycol additives have been used to prevent heat exchange coils from freezing. Although such additives may prevent frozen coils, they have reduced heat transfer coefficients and therefore require more pump energy. Dampers and high velocity jets have also been used to help in the mixing of air streams, but often the use of such devices creates unacceptable levels of pressure drop in the system. Specially designed air mixers have also been proposed in the past and these can improve the mixing of the air streams. However, these known mixers have some inherent defects which can be caused by the air streams being forced to pass through a narrow cross-section of the mixer. These known air mixers generally require more downstream space, can create a non-uniform downstream velocity profile and can cause a high pressure drop across the mixer. In addition, a non-uniform velocity profile caused by the air mixer can generate an extra pressure drop at downstream filter and coil sections.
In an attempt to increase the mixing that occurs in an air mixer, air mixers have been developed that create a great deal of turbulence between the incoming warm air and the incoming cold air. By increasing the turbulence, greater air mixing is achieved. However, such prior art air mixers simply use static baffles to create the turbulence in the air flow. As such, the amount of turbulence depends upon the volume of air flow. In conditions where there is a large temperature gradient and a small intake flow, such systems are no more effective than conventional air mixer configurations. Such prior art air mixers that use static baffles are exemplified by U.S. Pat. No. 6,139,425 to Yazici, entitled High Efficiency Air Mixer.
A need therefore exists for an improved air mixer that can mix two different sources of air at differing temperatures in a highly space efficient and energy efficient manner without having to depend upon air flow volume to create turbulence. This need is met by the present invention as described and claimed below.
The present invention is a mixing box for mixing two separate sources of air. The mixing box defines a mixing chamber. Air from a first source is fed into a plurality of vented conduits. Each of the vented conduits has at least one vent opening, that allows the air from within the conduits to escape and enter the mixing chamber. Air from a second air source flows into the mixing chamber around the vented conduits. As the second air source flows around the vented conduits, turbulence is produced that mixes the second air source with the first air source escaping from the vented conduits.
Flaps are used to control the flow of the first source of air out of the vented conduits and the flow of the second source of air around the vented conduits. The flaps are selectively positionable between a first position and a second position. When in a first position, the flaps restrict the flow of the first air source out of the vent openings in the vented conduits. When moved toward the second position, the same flaps no longer obstruct the flow of the first air source out of the vented conduits, but now obstruct the flow of the second air source around the vented conduits. By selectively controlling the position of the flaps, the relative flow of air into the mixing chamber from the first air source and the second air source can be dynamically controlled.